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United States Patent |
5,666,197
|
Guerra
|
September 9, 1997
|
Apparatus and methods employing phase control and analysis of evanescent
illumination for imaging and metrology of subwavelength lateral surface
topography
Abstract
Imaging and metrology devices employ controlled phase shifting and analysis
of evanescent light to provide enhanced ability to image and/or resolve
substantially subwavelength lateral features on a surface illuminated by
the evanescent light. The light waves comprising the evanescent
electromagnetic field are inhomogeneous in that their planes of equal
phase are substantially perpendicular to the direction of propagation and
to their planes of constant amplitude. The planes of equal phase are
therefore normal to the surface to which the evanescent field is adjacent
and to a sample surface illuminated by this field as well. By controlling
the phase of the source of illumination and analyzing the output from the
surface, either by phase analysis or phase to amplitude decoding,
subwavelength lateral surface topography resolution is enhanced without
sacrificing vertical resolution. Methods and means for dynamic or static
phase shifting of inhomogeneous waves comprising the evanescent field are
disclosed, as well as other non-imaging applications.
Inventors:
|
Guerra; John M. (Concord, MA)
|
Assignee:
|
Polaroid Corporation (Cambridge, MA)
|
Appl. No.:
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701173 |
Filed:
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August 21, 1996 |
Current U.S. Class: |
356/512; 977/DIG.1 |
Intern'l Class: |
G01B 009/02 |
Field of Search: |
356/345,357,359
|
References Cited
U.S. Patent Documents
4660980 | Apr., 1987 | Takabayashi et al. | 356/357.
|
Primary Examiner: Font; Frank G.
Assistant Examiner: Kim; Robert
Attorney, Agent or Firm: Stecewycz; Joseph
Claims
What is claimed is:
1. A phase controlled evanescent field imaging system for visualizing and
measuring subwavelength topographic features of a sample, said system
comprising:
lighting means for providing at least one beam of radiation having at least
one predetermined wavelength and phase angle;
optical means for receiving said beam of radiation from said lighting means
and creating at a surface a wavelength evanescent field having planes of
equal intensity parallel to said surface and planes of equal phase
substantially perpendicular to said surface so that light initially bound
within said wavelength evanescent field may be converted into propagating
light when the sample is brought proximate to said surface and such that,
in the absence of a sample near said surface, or if a sample surface is
distant from said surface, no evanescent light energy is converted into
propagating light, the phase and intensity of said propagating light
varying in correspondence with the topographic features of said surface
and their local proximity to said evanescent field;
phase control means for selectively controlling said predetermined
wavelength and phase angle of said beam of radiation such that the
intensity of said evanescent light varies as a function of the phase of
said beam of radiation and the lateral position of the local topographic
features of the sample;
imaging means having at least one optical axis positioned to collect said
propagating light and form images with it in which the topographic
features of the surface are encoded as imagewise variations in image
intensity in correspondence with the phase of said beam of radiation;
photodetector means for generating image signals having an amplitude that
varies in accordance with the imagewise variation in intensity in said
images; and
image processing means for receiving said image signals and generating a
composite image signal from which the topographic features of the sample
can be visually displayed or measured.
2. The system of claim 1 wherein said phase control means is dynamic in
operation.
3. The system of claim 1 wherein said phase control means is static in
operation.
4. The system of claim 1 wherein said phase control means comprises
filtering means for selectively controlling the output of said lighting
means to preselected wavelengths of radiation.
5. The system of claim 4 wherein said filtering means is structured to pass
at least two different wavelengths.
6. The system of claim 1 wherein said phase control means comprises
polarization means.
7. The system of claim 1 wherein said phase control means comprises an
electro-optic modulator.
8. The system of claim 1 wherein said phase control means comprises a
tunable optical cavity.
9. The system of claim 1 wherein said phase control means comprises means
for changing the angle of incidence of said beam of radiation from said
lighting.
10. The system of claim 1 wherein said phase control mean comprises a
retarding phase plate.
11. The system of claim 10 wherein said retarding phase plate comprises
annular phase masks with different numerical apertures greater than one.
12. The system of claim 1 wherein said phase control means comprises a
split polarizer.
13. The system of claim 1 wherein said phase control means comprises a
split color filter.
14. The system of claim 1 wherein said phase control means comprises means
for selecting the azimuthal angle of the incident illumination, and thus
the direction of the Goos-Haenchen shift.
15. The system of claim 1 wherein said optical means comprises a tapered
dielectric near-field probe with subwavelength aperture to increase
lateral resolution.
16. The system of claim 15 wherein said near-field probe is formed of a
multiple dielectric layer Bragg-type reflector having an aperture where
the evanescent field is restricted in the lateral plane.
17. The system of claim 16 wherein said probe and Bragg reflector are
formed together by drawing a preform containing the Bragg layers scaled up
to size.
18. The system of claim 1 wherein said optical means comprises a plurality
of probes arranged in a close-packed matrix for locally scanning over a
small distance so that lateral phase shifting is achieved in addition to
increasing total scan area and reducing scan time.
19. The system of claim 18 wherein said close-packed matrix of said
plurality of probes is arranged to comprise a phase-array so that together
said probes form a synthetic aperture.
20. The system of claim 1 wherein said optical means comprises a
diffractive grating structure having a spatial grating period smaller than
the wavelength of said beam of radiation from said lighting means such
that the diffracted orders are evanescent where the local phase of the
evanescent field is determined by and phase-locked to said diffractive
grating structure.
21. The system of claim 20 further including means for varying said spatial
grating period of said diffractive grating structure to shift the phase of
the evanescent field.
22. The system of claim 21 wherein said means for varying said spatial
grating period comprises a tunable acoustic wave modulator.
23. The system of claim 21 wherein said means for varying said spatial
grating period comprises a piezo crystal modulator.
24. The system of claim 1 wherein said optical means comprises a light
transmissive body having an entrance facet for receiving said beam of
radiation from said lighting means from which said evanescent field is
formed, a distal facet at which said evanescent field is formed so that a
sample surface can be brought proximate to it, and at least one exit facet
from which propagating light emerges.
25. The system of claim 24 where said light transmissive body, said
entrance facet and said exit facet thereof are structured and arranged
with respect to one another so that said distal facet is a totally
internally reflecting barrier where said evanescent field is caused by
photons tunneling beyond said distal facet.
26. The system of claim 1 wherein said propagating radiation is specularly
reflected.
27. The system of claim 1 wherein said propagating radiation is scattered.
28. A method for visualizing and measuring subwavelength topographic
features of a sample utilizing phase controlled evanescent field
illumination, said method comprising the steps of:
providing at least one beam of radiation having at least one predetermined
wavelength and phase angle;
receiving said beam of radiation and creating at a surface a wavelength
evanescent field having planes of equal intensity parallel to said surface
and planes of equal phase substantially perpendicular to said surface so
that light initially bound within said wavelength evanescent field may be
converted into propagating light when the sample is brought proximate to
said surface and such that, in the absence of a sample near said surface,
or if a sample surface is distant from said surface, no evanescent light
energy is converted into propagating light, the phase and intensity of
said propagating light varying in correspondence with the topographic
features of said surface and their local proximity to said evanescent
field;
selectively controlling said predetermined wavelength and phase angle of
said beam of radiation such that the intensity of said evanescent light
varies as a function of the phase of said beam of radiation and the
lateral position of the local topographic features of the sample;
collecting said propagating light and forming images with it in which the
microtopographic features of the surface are encoded as imagewise
variations in image intensity in correspondence with the phase of said
beam of radiation;
generating image signals having an amplitude that varies in accordance with
the imagewise variation in intensity in said images; and
receiving said image signals and generating a composite image signal from
which the topographic features of the sample can be visually displayed or
measured.
29. The method of claim 28 wherein said step of selectively controlling the
wavelength and phase of said beam of radiation is dynamic in operation.
30. The method of claim 28 wherein said step of selectively controlling the
wavelength and phase of said beam of radiation is static in operation.
31. The method of claim 28 wherein said step of controlling the wavelength
and phase of said beam of radiation comprises filtering said beam of
radiation to restrict its output to preselected wavelengths of radiation.
32. The method of claim 28 wherein said step of controlling the wavelength
and phase of said beam of radiation comprises polarizing said beam of
radiation.
33. The method of claim 28 wherein said step of controlling the wavelength
and phase of said beam of radiation comprises changing the angle of
incidence of said beam of radiation from said lighting means.
34. The method of claim 28 wherein said step of controlling the wavelength
and phase of said beam of radiation comprises retarding said beam of
radiation.
35. The method of claim 28 wherein said step of controlling the wavelength
and phase of said beam of radiation comprises selecting the azimuthal
angle of the incident illumination, and thus the direction of the
Goos-Haenchen shift.
36. The method of claim 28 wherein said step of receiving and creating said
evanescent field comprises illuminating a diffractive grating structure
having a spatial grating period smaller than the wavelength of said beam
of radiation from said lighting means such that the diffracted orders are
evanescent where the local phase of the evanescent field is determined by
and phase-locked to said diffractive grating structure.
37. The method of claim 36 further including the step of varying said
spatial grating period of said diffractive grating structure to shift the
phase of the evanescent field.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention in general relates to the fields of imaging and metrology of
surfaces and, more specifically, to systems and methods for providing
controlled phase evanescent field illumination for visualizing, imaging,
energizing, and measuring submicron topographic features with enhanced
lateral resolution.
2. Description of the Prior Art
The use of evanescent fields for measuring and visualizing submicron
surface topographic features is known. Descriptions of evanescent field
usage are contained, for example, in: Harrick, N.J., "Use of Frustrated
Total Internal Reflection to Measure Film Thickness and Surface Reliefs,"
J. Appl. Phys., 1962. 33: p. 321; McCutchen, C. W., "Optical Systems for
Observing Surface Topography by Frustrated Total Internal Reflection and
by Interference," The Review of Scientific Instruments, Vol. 35, p.
1340-45, 1964; Guerra, J. M., September 1988, "Photon tunneling
microscopy," in Proceedings from Surface Measurement and Characterization
Meeting, Hamburg, SPIE Vol. 1009, pp. 254-62; U.S. Pat. No. 4,681,451,
entitled "Optical proximity imaging method and apparatus," issued 21 Jul.
1987 to Guerra, J. M. and Plummer, W. T.; U.S. Pat. No. 5,349,443 entitled
"Flexible transducers for photon tunneling microscopes and methods for
making and using same," issued 20 Sep. 1994 to Guerra, J. M.; and, U.S.
Pat. No. 5,442,443 entitled "Stereoscopic Photon Tunneling Microscope,"
issued 15 Aug. 1995 to Guerra, J. M., all patents assigned to Polaroid
Corporation.
Harrick, McCutchen, Guerra/Plummer, and Guerra disclose whole-field
reflected evanescent light microscopes where the sample is neither
transilluminated nor scanned, but is rather illuminated by an evanescent
field from an unrestricted total reflection surface at the object plane of
an epi, or reflected-light, illuminator. Here, the sample can be opaque or
transparent, thick or thin, and can be viewed in real-time with high
energy throughput. Such microscopes are very sensitive to smooth surfaces
because of their use of the exponentially varying amplitude of the
evanescent field in the vertical direction to sense very small surface
height variation. On the other hand, rougher surfaces scatter light back
into the microscope, which decreases contrast and sensitivity. Also, the
deeper topography is rendered as bright, because these areas penetrate the
evanescent field to a small degree so that the epi-illumination is nearly
totally reflected. The difficulty in detecting and measuring small changes
in bright scenes limits the observable topographic depths to about 3/4 of
the illuminating wavelength (which is the wavelength in air divided by the
index n and the sine of the incident angle I). Further, the illumination
and imaging optics are coupled because the objective element also serves
as the condenser. In a practical sense, this limits the use of such
instruments to the availability of suitable commercial objectives,
magnifications, fields of view, and numerical aperture. In addition, it is
difficult, because of the coupling of imaging and illumination optics, to
affect the polarization, phase, incident angle, and direction of the
illumination. This, in turn, restricts the ability to maximize the
tunneling range, increase lateral resolution, or tunnel through less rare
media such as water in biological applications.
Devices in which evanescent light from transilluminated samples is
scattered into objective pupils are described in: G. J. Stoney,
"Microscopic Vision," Phil. Mag. 332, at 348-9, 1896: Surface contact
microscope, Taylor & Francis; Ambrose, E. J., "A Surface Contact
Microscope for the Study of Cell Movements," Nature, Nov. 24, 1956, Vol.
178; Ambrose, E. J., "The Movements of Fibrocytes,"Experimental Cell
Research, Suppl. 8, 54-73 (1961); Temple, P. A., "Total internal
reflection microscopy: a surface inspection technique," Applied Optics,
Vol. 20, No. 15 Aug. 1981; and, D. Axelrod, in Fluorescence Microscopy of
Living Cells in Culture, Part B, ed. D. L. Taylor and Y-L. Wang, (Academic
Press, New York, 1989), Chap. 9.
Stoney, Ambrose, Temple, and Axelrod disclose optical evanescent light
field microscopes in which the light that enters the objective pupil is
evanescent field light that has been scattered from a sample surface.
However, in all of the microscopes described in the above references, the
sample is transilluminated with the illumination incident at beyond the
critical angle such that the evanescent field from the sample surface is
received. In these applications, it is a requirement either that the
sample material be transparent at optical frequencies or that the sample
itself be thin enough to be transparent.
Scanning devices which rely on scattered evanescent field light are
described in Fischer, U. Ch., Durig, U. T., and Pohl, D. W., "Near-field
optical scanning microscopy in reflection," Appl. Phys. Lett., Vol. 52,
No. 4, pp. 249-51, 25 Jan. 1988. Fischer et al. disclose a near-field
optical microscope in which the sample is not transilluminated but is
rather illuminated in reflected light. Further, this reflected light is in
the form of an evanescent field from a dielectric plate into which light
is launched at greater than the critical angle, by means of a coupled
prism, so that it undergoes multiple total internal reflections, giving
rise to the evanescent field. However, Fisher et al restrict the
evanescent field with an aperture in a metal opaque coating on the total
reflection surface of the dielectric plate. This aperture is smaller than
the light wavelength so that an improvement in lateral resolution beyond
the normal Abbe limit is achieved, but at the cost of having to scan the
aperture relative to the sample to build up an image. A further cost is
that energy throughput is very low, making extension to analytical optical
techniques such as spectroscopy problematic.
Devices which utilize transillumination of transparent samples are
described in R. C. Reddick, R. J. Warmack, and T. L. Ferrell, "New form of
scanning optical microscopy," Phys. Rev. B, 39, 767-70 (1989). Reddick et
al. discloses transillumination of thin and transparent samples with
evanescent light, but the entrance pupil in Reddick et al is not an
objective in the conventional microscopy sense. Rather, it is an optical
fiber that is scanned over the sample, close to the sample surface. Thus,
there is a loss of flux throughput, and vertical resolution is limited by
the mechanism that controls the vertical position of the fiber relative to
the sample. A means of scanning in the xy plane is also required,
preventing true real-time whole-field imaging.
In addition to the use of evanescent fields and scattering for imaging and
other purposes, phase shifting interferometry has played important roles
in different contexts. For example, the wave nature of light has been
beneficially employed in optical microscopy where vertical height
resolution is limited to .lambda./2. Here, interference between wavefronts
can be employed to increase vertical resolution and contrast. Interference
between wavefronts with a static, or fixed phase shift as in differential
interference contrast microscopy, invented by Nomarski (G. Nomarski, J.
Phy. Rad. 16, 9S (1955)), or phase contrast microscopy invented by Zernike
(1935), results in a contrast enhancement so that normally unresolved,
substantially subwavelength vertical differences are made visible.
Typically, the unifying principle behind the many manifestations of
interferometers is that a reference wavefront is made to interfere with an
unknown. In the last case, the wavefront is combined with a phase-shifted
version of itself.
In other interferometers, a controlled, known reference wavefront is split
into two wavefronts. One is disturbed by the sample, and the disturbed
wavefront is recombined with the reference. The resulting interference
image, or interference map, is analyzed to determine vertical information
about the disturbing sample surface.
In all manifestations, phase can be measured to better than one part in one
hundred of the wavelength, .lambda.. This high resolution, well beyond the
Abbe limit, is termed superresolution, but is only in the vertical axis.
Spatial resolution in the XY plane remains at best .lambda./2.
Interference microscopes such as have been available commercially from WYKO
and ZYGO also achieve vertical resolution of .lambda./100, or better,
through dynamic phase shifting with, for example, a piezo-actuated
reference window. The phase shifting causes a multitude of sequential
interference images, each the result of a discrete and unique phase shift,
in which the interference information is manifested as an amplitude or
light intensity variation in the spatial image plane. While the data from
each of the multitude of interference images, or maps as they are
sometimes called, must be reduced to obtain the final image, the
resolution is remarkable and, unlike Nomarski and phase contrast,
quantitative. The shifting must be over at least one complete fringe in
order to extract full information. The vertical range of this phase
shifting technique is about half the wavelength of the illumination.
While the art describes a variety of devices that utilize evanescent field
illumination for investigating vertical surface characteristics, there
remains a need for improvements that offer advantages and capabilities not
found in presently available instruments, and it is a primary object of
this invention to provide such improvements.
It is another object of the invention to apply methods and means of phase
shifting and phase shifting interferometry to the phase of the
inhomogeneous waves comprising evanescent fields to achieve
superresolution in the lateral spatial plane.
Another object is to employ lateral phase shifting to achieve
superresolution imaging in optical systems using evanescent field
illumination, while maintaining whole field (rather than scanning) imaging
as in the photon tunneling microscope.
Another object is to control the phase of the evanescent field by
illuminating a diffractive structure with a spatial grating period smaller
than the illumination wavelength so that the evanescent modes are phase
locked to the diffracting structure, and then to provide means to modify
the grating period in order to shift the evanescent field phase.
Other objects of the invention will be obvious, in part, and, in part, will
become apparent when reading the detailed description to follow.
SUMMARY OF THE INVENTION
Imaging and metrology devices employ phase shifting and analysis of
evanescent light to provide enhanced ability to image and/or resolve
substantially subwavelength lateral features on a surface illuminated by
the evanescent light. The light waves comprising the evanescent
electromagnetic field are inhomogeneous in that their planes of equal
phase are substantially perpendicular to the direction of propagation and
to their planes of constant amplitude. The planes of equal phase are
therefore normal to the surface to which the evanescent field is adjacent
and to a sample surface illuminated by this field as well. By controlling
the phase of the source of illumination and analyzing the output from the
surface, either by phase analysis or phase to amplitude decoding,
subwavelength lateral surface topography is enhanced without sacrificing
the detail of vertical features. Methods and means for dynamic or static
phase shifting of inhomogeneous waves comprising the evanescent field are
disclosed, as well as other non-imaging applications.
In accordance with a further feature of the invention, the phase shift and
analysis can be static (constant) or dynamic.
In accordance with other features of the invention, the phase shift and
analysis is achieved by control and selection of illumination wavelength,
use of two wavelengths, polarization state control including electro-optic
modulation, incident angle, azimuthal incident angle, optical cavity
control, and phase retardation, where control in all embodiments may be
for either unidirectional or omnidirectional incident illumination.
In accordance with yet another feature of the invention, the phase shift
and analysis are adapted to a photon tunneling microscope for
superresolution lateral imaging and metrology for use in, for example, the
semiconductor industry or biological research.
In accordance with yet another feature of the invention, the means of
controlling and shifting the phase of the evanescent field is a
diffractive structure with variable grating period substantially less than
the illumination wavelength, where the phase of the evanescent modes thus
formed is locally locked to the diffraction structure, and the variable
grating period can be tuned.
In accordance with yet another feature of the invention, the means and
methods of phase shifting are adapted to the evanescent field from highly
bound waves, such as at the tip of an optical scanning probe that is an
optical waveguide operated below its cutoff frequency to reduce the
effective aperture of the probe and thus increase lateral resolution.
In accordance with yet another feature of the invention, the means of
controlling the phase of the evanescent field surrounding the core of an
optical waveguide or scanning probe is in the form of a Bragg reflector
stack of dielectric layers, such that the effective aperture of the probe
is reduced over the normal metallic aperture.
In accordance with yet another feature of the invention, means to control
and shift the phase of the evanescent field is a plurality of close-packed
tapered waveguides operated below their cutoff frequency, where the phase
of the resulting evanescent field is locked to the individual waveguides,
with additional means to locally scan the plurality of probes laterally
over a small distance.
In accordance with yet another feature of the invention, the plurality of
close-packed tapered waveguides are arranged to form a phase-array
synthetic aperture.
Other features of the invention will be readily apparent when the following
detailed description is read in connection with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
The structure and operation of the invention, together with other objects
and advantages thereof, may best be understood by reading the detailed
description to follow in connection with the drawings in which unique
reference numerals have been used throughout for each part and wherein:
FIG. 1 is a diagrammatic partially elevational and partially perspective
view of an imaging and metrology system of the invention employing a phase
controlled evanescent field illumination section;
FIG. 2 is a flow chart illustrating the operation of the of the invention
of FIG. 1;
FIG. 3 is a diagram showing how the amplitude and position of an evanescent
field is modified as a result of a phase change;
FIG. 4 is a diagram showing how a conventional phase shift, .delta..sub.p,
results in an axial shift in propagating light and thus height
information;
FIG. 5 is a diagram showing how a phase shift, .delta..sub.p, results in a
lateral shift in evanescent field light and thus spatial information in
the XY plane;
FIG. 6 is a diagram showing a propagating wavefront incident on a total
reflection surface, showing the phase shift caused by a device inserted
into the wavefront, with the device shown in elevation as a cross-section;
FIG. 7 is a diagrammatic planar view showing the phase shifting device of
FIG. 6;
FIG. 8 is a diagrammatic elevational view showing the phase shifting device
of FIGS. 6 and 7 in combination with a prism and sample;
FIG. 9 is a diagrammatic planar view of a phase shifting plate for
insertion in the optical path upstream of the prismatic element of FIG. 1;
FIG. 10 is a diagrammatic planar view of a linear polarizer for use in
controlling phase shifting such as in the system of FIG. 1;
FIG. 11 is a diagrammatic view of an image area;
FIG. 12 is a diagrammatic view illustrating how angle of incidence can be
used, either statically or dynamically, to control phase of evanescent
field illumination;
FIG. 13 is a diagrammatic planar view of a mask for selecting specific
incident angles as shown in FIG. 12;
FIG. 14 is a diagrammatic view showing how incident angle may be selected
when used in a prism arrangement;
FIG. 15 is a diagrammatic view of phase shift accomplished by varying the
optical path by physical axial movement as, for example, control by a
piezo actuator;
FIG. 16 is a diagrammatic elevational view of an evanescent field born of a
subwavelength diffracting structure, with phase determined by structure
geometry and with the structure dynamically adjustable for tuned
heterodyning phase control;
FIG. 17 is a diagrammatic elevational view of the evanescent field geometry
in a near-field probe, showing how only the center of the field has equal
phase planes normal to the probe so that phase shifting and restriction
results in a smaller effective aperture and higher resolution;
FIG. 18 is a diagrammatic elevational view of an improved near-field probe
where the aperture is a multi-layer stack Bragg reflector;
FIG. 19 is a partial diagrammatic perspective view of phase shifting matrix
of probes moveable in the XY plane;
FIG. 20 is a diagrammatic elevational view of a system that employs phase
controlled evanescent field illumination for reading and writing data in
the optical domain to provide enhanced information storage densities in
optical media via increased lateral spatial resolution; and
FIG. 21 is a diagrammatic elevational view of an aplanatic sphere that may
be used in the system of FIG. 20.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
This invention generally relates to the fields of surface imaging and fine
detail measurement of subwavelength topography through the application of
phase controlled evanescent field illumination. In particular, the
invention relates to methods and apparatus by which phase controlled bound
evanescent field illumination is converted by frustration of total
internal reflection and/or scattering into propagating light that is
subsequently collected and imaged for downstream display and metrology
purposes through the use of analysis and image processing techniques for
decoding the phase encoded information contained in the propagating
radiation. A number of different phase control evanescent field
illumination embodiments are described which have utility in their own
right but are also combined with other inventive features for imaging,
metrology, and other purposes, as will become evident.
Reference is now made to FIG. 1 which shows generally at 10, a phase
controlled evanescent field imaging system of the invention. As will be
seen, system. 10 is particularly suitable for full-field, video
visualization and measurement of microtopographic features of surfaces
having surface characteristics ranging from smooth to rough or
combinations of partially refracting/diffracting (specular) to partially
scattering (rough), or, more generally, to any optical properties (e.g.,
micro spatial index variations) capable of converting bound evanescent
fields to propagating radiation. As will be further seen, system 10
collects propagating light in specular form for smoothness analysis and in
scattered form to assess rougher surface characteristics. In assessing
smoother surfaces, images appear as dark against a light background and in
the assessment of optically scattering surfaces, images are light against
a dark background or are "dark field" images. System 10 and other
embodiments of the invention are suitable for use in measuring the surface
characteristics of aircraft and space vehicle structures, paint, paper,
and fabrics. Because of the lighting and detecting mechanisms employed in
dark field imaging, it is possible to decouple the illumination and
imaging paths so that probes remote from the imaging sections can be
contrived.
As seen in FIG. 1, system 10 comprises an illumination and image formation
section 12 and an image processing and display section 14. Section 14
comprises any well-known general purpose computer or work station 16
having a CPU, RAM memory, hard and floppy drives, input devices such as a
keyboard 18 and mouse 20, and color display monitor such as at 22.
Preferably, computer 16 has at least sixteen megabytes of RAM and is
otherwise equipped with high throughput data and video buses. The internal
video card is preferably one selected with at least two megabytes of
on-board memory and is capable of generating 32-bit or more colors for
high tone resolution. Internal signal and image processing programs are
stored on the internal hard drive of computer 16 and transferred to RAM in
the usual way for processing needs as required.
As best seen in FIG. 1, illumination and imaging section 12 comprises an
optical head 24 for facilitating illumination of and proximity with the
surface of a sample to be visualized and measured, and video cameras 26
and 27 for collecting and imaging propagating radiation from the surface
of a sample.
Video cameras 26 and 27 may be any suitable conventionally available type
of the desired spatial and tonal resolution. Preferably, each of video
cameras 26 and 27 is a wide-angle type and may have a magnifying or
slightly minifying objective lenses 28 and 29, respectively. Objective
lenses 28 and 29 each may alternatively be a zoom lens of appropriate
tele- to wide-angle design. Alternatively, video cameras 26 and 27 may be
directed into a microscope.
Located at the plane of best focus of objective lens 28 is a photo detector
30, which may be a conventional CCD or vidicon tube. Video signals
generated from photo detector 30 are digitized on board camera 26 via a
resident chip 32 for that purpose or sent to an appropriate board resident
in a slot in computer 16.
Located at the plane of best focus of objective lens 29 is a photo detector
31, which also may be a conventional CCD or vidicon tube. Video signals
generated from photo detector 31 may be processed on board camera 27 via a
resident chip 33 for that purpose or may be sent to an appropriate board
resident in a slot in computer 16.
In either event, cameras 26 and 27 and computer 16 are configured in
well-known manners so that video signals are digitized to generate digital
images that are displayed on monitor 22 at video rates or nearly video
rates as image processing speed permits.
Optical head 24, in one preferred embodiment, comprises a bulk optic,
prismatic dielectric body 34, having, among others, a light entering facet
42, a sample proximity facet 44 for contacting an adjacent surface of a
sample such as that indicated at 45, and light emitting facets 46 and 47.
Alternatively, optical head 24 may be an aplanatic sphere. Optically
coupled to facet 42 is a phase controlled illumination section 40
comprising a collimating optical section 36 consisting of a tube in which
are resident suitable collimating optics in the form of a well-known bulk
optic lens system shown diagrammatically at 38. An illumination source 39
is provided, and radiation emitted by source 39 is directed via
collimating optical section 36 through a phase controller 41 that acts as
a means for controlling the phase of illumination directed toward optical
head 24. Illumination source 39 preferably is of narrow bandwidth and may
comprise an LED, laser, or other source provided with narrow bandpass
filters to limit wavelength to predetermined ranges.
As will be seen subsequently, phase controller 41 and its functions may be
provided by a number of different means and is preferably under the
control of computer 16.
FIG. 2 is a preferred flow chart for the general operation of imaging
system 10. As shown, illumination source 39 first provides a beam of
radiation of predetermined wavelength, phase, and numerical aperture as
designated in block 50. The phase of the beam of radiation is then
selectively controlled as indicated in block 52; and the phase is
communicated to block 62, which also provides phase change control signals
to block 52 to selectively modulate the phase as required. Once the phase
is set, the phase controlled illumination enters optical head 24 to create
an evanescent field that is bound to proximity surface 44 in a manner to
be described. The bound phase controlled evanescent field is then
reflected, refracted, diffracted, or scattered as propagating radiation
(block 56) when the near surface of a sample 45 is brought into near
proximity surface 44. Most sample surfaces will exhibit more or less
refracted/diffracted or scattered radiation in accordance with their
smoothness/roughness properties. When a surface is very smooth, it will
predominantly reflect radiation and, when optically scattering, it will
scatter radiation. Propagating radiation from smoother surfaces originates
from frustrated total internal reflection at the boundary of contact
surface 44 and the contacting sample surface. This propagating radiation
is designated generally at 53 (See FIG. 1).
Scattered radiation is generally designated at 51 and propagates over a
solid angle dictated by the properties of the sample surface. Specular
propagating radiation is analyzed by a phase analyzer 76 (FIG. 1) and
imaged by video camera 27 while propagating scattered radiation is
analyzed by a phase analyzer 78 and imaged via video camera 26, all under
the control of computer 16 as in block 58. The intensity of the
propagating radiation varies as a function of the phase of the
illuminating beam and the lateral position of local subwavelength
topographic features of the sample's surface, and one could directly
visualize the image from block 58 with the aid of a microscope.
For each phase of controlled radiation, the corresponding propagating
radiation is analyzed and one or more image signals are generated in block
60 in which the amplitude of the signal varies in accordance with the
imagewise variation in and in correspondence with the phase of the
illuminating beam of radiation. It should be clear that either or both
types of propagating radiation can be considered, and this may be done
simultaneously or separately.
The image signal(s) is encoded as in block 62 as a gray tone map in which
tone levels indicate lateral and vertical features as a function of phase.
This may be via a look-up table (LUT). The encoded image signals are then
processed to form a composite signal in block 64 and this information can
be stored in memory in block 66 or printed out as data in hard copy form
in block 68.
The gray tone map signal is then formatted for 2D and 3D display via
computer 16 or other suitable dedicated image processor. The composite
formatted signal is then fed to a video processor in computer 16 from
which it can be displayed on monitor 22 as shown block 72.
The amount of signal processing that occurs in block 64 to produce the
composite image signal can vary depending on the task at hand. It can be
as simple as subtracting two gray tone maps generated at different phases
to provide an image in which edges will be enhanced or can be a complex
reduction of a series of sequential interference images in which
interference information is manifested as an amplitude or light intensity
variation in the spatial domain.
To better understand how phase control and analysis of the evanescent
illuminating field operates to achieve enhanced resolution in the lateral
spatial plane, reference is now made to FIGS. 3 and 5.
As best seen in FIG. 5, the light transmissive dielectric body 34 of
optical head has light traveling through at an incident angle equal to or
greater than the critical angle, .theta..sub.c, so that it totally
internally reflects at the interface with a less dense medium such as air
or water, i.e., at facet 44. It is understood that dielectric optical head
24 can be transmissive over all or any part of the electromagnetic
spectrum, including but not limited to ultra-violet, infra-red, or even
x-ray and millimeter wave extremes, depending on the application. Although
the incident light is shown to preferably be a collimated beam, it need
not be, and in fact imaging resolution improves if the incident light is
less coherent.
As seen in the left of FIG. 3, an evanescent field 80 at a first phase
angle, .phi..sub.1, arises at the boundary between facet 44 and the lower
index medium opposite it (usually air, but this medium may also be water
or another low index medium). Evanescent field 80 has an amplitude that
decays exponentially with distance from the surface of facet 44. The
strength of evanescent field 80 is given by:
##EQU1##
where E.sub.0 is the phase dependent amplitude of the electric field
associated with the photon in the medium comprising optical head 24 and,
d.sub.p, is the penetration depth in the less dense medium at which
E.sub.0 decreases to E.sub.0 /e and where:
##EQU2##
and .lambda..sub.1 is the wavelength in the denser medium, .theta. is the
incidence angle, and n.sub.21 in the ratio of denser to lower indices of
refraction at the boundary of facet 44. The actual penetration depth,
where E.sub.evanescent falls to the limit of detectability, is dependent
on these variables as well as both the photodetector sensitivity and the
sample optical properties, and is typically approximately 0.75. However,
the evanescent field, however small in intensity, can exist sensibly for
tens of wavelengths, if the parameters in equation (2) are optimized.
As is well-known, evanescent field 80 penetrates normal to the surface of
facet 44 to the depth indicated above. Consequently, it extends beyond the
physical boundary of facet 44 to a predetermined depth and can be
interrupted by a sample placed in close proximity to facet 44.
The amplitude of evanescent field 80 at the proximity facet 44 is E.sub.01
and is phase angle dependent since the standing wave, E.sub.0, is a
superposition of the incident and reflected waves at the total reflection
interface (the boundary between high and low index media) and has an
electric field amplitude, E.sub.0, where
##EQU3##
that is, twice the amplitude of the standing wave at the given phase from
addition of the incident and reflected wave and has an evanescent
wavelength .lambda..sub.e,
##EQU4##
where .lambda..sub.1 is .lambda./n.sub.1, the free space wavelength
divided by the refractive index of the medium 1 (in this case, the denser
medium of the optical head 24). The standing wave and the penetrating wave
are normal to the surface.
Referring now to the right side of FIG. 3, it can be seen that there is
another evanescent field 82 whose amplitude, E.sub.02, at the proximity
facet 44, is smaller than that of E.sub.01 in accordance with equation (3)
where that phase angle is now .phi..sub.2. Also notice that the change in
phase angle has shifted the evanescent field laterally in x from position
x.sub.0 to position (x.sub.0 +.DELTA.x). This lateral shift coupled with
the reduction in amplitude with phase is an important property of phase
controlled evanescent field illumination that is exploited to advantage by
this invention. It should also be noted that with evanescent fields, there
are planes parallel to the proximity facet 44 that represent planes of
equal amplitude at a particular phase, while planes of equal phase are
perpendicular to the planes of equal amplitude and laterally positioned as
a function of phase angle value.
The relationship between the planes of constant phase and planes of
constant amplitude for propagating light versus the evanescent field
arising from tunneling light, in this case from refraction beyond the
critical angle, is illustrated in FIG, 5. Radiation (85-87) is incident at
greater than the critical angle, .theta..sub.c, from the optical axis,
O.sub.A. Now, the proximity facet 44 totally internally reflects until the
sample surface (45) penetrates the resulting evanescent field indicated
generally at 89. The planes of equal phase shown at 88 are perpendicular
to both the proximity facet 44 of optical head 24, the sample surface, and
planes of equal amplitude 90, so that a phase difference, .delta..sub.p,
results in a lateral shift in the evanescent field light and thus is able
to yield spatial information in the XY plane (Y is into the paper). As the
incident angle increases for incident light waves (87), (86), and (85),
respectively, the penetration of the boundary (44), and thus the amplitude
of the evanescent field, decreases. There is also a lateral phase shift
between these wavefronts because of the difference in incident angle, but
this is not shown for clarity. As the phase of the incident illumination
changes, the constant phase planes of the illumination sweeps across the
sample surface features in the XY-plane. As it does so, the phase of the
propagating radiation is modulated by just the lateral features of the
sample since these features are the only source by which the propagating
radiation may be modulated under these conditions.
Reference is now made to FIG. 4 which, by contrast, shows the phase and
amplitude relationships that hold in a normal propagating optical
microscope. Here, the planes of equal phase 100 and equal amplitude are
parallel and a phase shift, .delta..sub.p of the propagating incident
light 102 in, for example, an optical light, microscope, results in an
axial shift in the propagating light 102 as seen by the surface of sample
45, where the reference surface may be that of an objective or prismatic
optical head, whether dry or oil immersion, as indicated by the solid line
104. The planes of constant phase 100 are coplanar with the planes of
constant amplitude (not shown) and are perpendicular to the propagation
direction which for illustration is shown to be along the optical axis,
O.sub.A. In this case, which is the basis for traditional interferometry,
phase information can yield only optical path measurement, which when
applied to metrology of a sample surface, yields only height, and not
lateral, information.
By way of further explanation, FIG. 6 shows a generic propagating wavefront
110, shown incident on a total reflection (TIR) surface 112 at greater
than the critical angle, .theta..sub.c. The incident light has wavelength,
.lambda., and amplitude, E.sub.0. Planes of constant phase 114 are normal
to the propagation direction, and a phase differential .delta..sub.p is
shown. A generic phase control device 116 (as 41 in FIG. 1) inserted into
the path of wavefront 110 shifts the phase of part of the wavefront 110 to
provide phase shifted wavefront 118 that has passed through a segment of
device 116 labeled 120. The phase of wavefront 118 is thus controllably
shifted relative to the wavefront that passes through a segment labeled
122. Device 116 is shown here in a highly schematic and symbolic form,
where the actual device can comprise a well-known differential phase
plate, a differential polarizer, a differential color filter, or other
phase-affecting optical device. The differential in phase between 120 and
122 is also rendered symbolically here. The actual geometry of the
boundary between the areas of shifted phase can be annular, opposing
slots, or other geometry as will be seen. Also, device 116 as symbolically
shown represents both static and dynamic phase shifting devices, with the
latter including, but not limited to, tunable optical cavities,
electro-optic modulators, piezo-actuated reference plates, phase conjugate
mirrors, or other. Some of these will be shown and discussed subsequently.
Device I 16 is shown in combination with a total internal reflection prism
such as the prismatic optical head 24 and sample surface 130, where the
incident light 110 is phase shifted by device 116, and the exiting light
132 is then phase analyzed through, for example, interferometry or
polarimetry in order to extract the lateral phase information about
surface 130 as encoded propagating radiation 132. The phase analyzer may
be either 76 or 78 as shown in FIG. 1.
Interferometry through phase shifting of the evanescent field as described
here benefits any optical device that employs evanescent illumination.
Other examples to be subsequently discussed include near-field proximity
masks, near-field scanning microscopes, the photon scanning optical
microscope, and the photon tunneling microscope disclosed in the
referenced patents above. With phase control, the photon tunneling
microscope enjoys lateral resolution approaching, through phase
measurement, its high vertical resolution, which is attained through
amplitude measurement of the decaying evanescent field, because the phase
of light can be measured to up to a hundredth part of the wavelength. The
nonscanning, high speed acquisition, whole field image is retained while
the lateral resolution is typical of a scanning optical near-field probe
microscope.
For example, the phase of the evanescent light can be shifted by: i)
changing the incident angle of illumination by some amount (where the
angle is already beyond the critical angle for total reflection), or ii)
keeping the incident angle fixed but selecting a particular polarization.
A sweeping fringe can be created accordingly in method (i) by interference
between two coherent light sources separated by some angle, where the
separation angle is variable and one of the sources is fixed, or the
separation angle is fixed and the sources are moved together angularly. In
method (ii), a sweeping interference fringe is generated between two
polarizations of the light source. This is best done at the principal
angle, where the phase difference between the parallel and perpendicular
polarizations is at a maximum. The polarization of the incident
illumination, if the illumination is omnidirectional, can be controlled
with a rotated Brewster angle polarizer or a tourmaline plate cut so that
it passes light polarized at right angles to its optical axis, or a
similar device.
In both cases the fringe is swept along the sample and is modulated by the
lateral variations in the sample. The subwavelength lateral variations are
revealed by the deviation of the fringe from the reference fringe with
phase angle, for example, .phi. of equation (3) set to zero. With this
technique, the lateral resolution is determined by how fine the phase can
be controlled, or resolved, and this is substantially less than the
wavelength and Abbe resolution limit.
Another phase shift occurs at the NA=1 junction, where the light incident
at greater than the critical angle, after being totally internally
reflected, is shifted by .pi., while the specularly reflected light at
less than the critical angle is shifted to a degree varying with the
incident angle. This can be a static or a dynamic shift.
The above discussion has largely assumed a collimated to weakly converging
beam, so that the incident illumination waves are almost planar. However,
the same invention applies to more strongly converging, highly bound beams
as well. Such beams can be the result of refraction in very high numerical
aperture optics, but are also caused by placing a subwavelength diameter
aperture in the total reflection surface. Apertures that are smaller than
the illumination wavelength are used in so-called near-field optical
scanning microscopes to increase lateral spatial resolution. However, even
these microscopes can benefit from the lateral phase shifting disclosed
here because the aperture size, and thus resolution, is ultimately limited
by the finite transmission of the aperture itself.
In addition, a plurality of such probes, as will be seen, arranged in an
array can scan a very small area, but the knitting together of these small
scans, with the scanning done in parallel, results in very fast image
acquisition. The scanning motion itself is at the same time shifting the
phase planes along the sample surface. Further, the plurality of probes
can be arranged to form a phased array synthetic aperture.
Another property when the incident illuminating wavefront is non-planar is
that energy flows in the XY plane in the evanescent field. This results in
a beam shift upon reflection, known Goos-Haenchen or G-H shift. Therefore,
selecting the azimuthal direction of the incident light rotates the G-H
shift, and is a form of phase-shifting unique to evanescent light. Also,
the choice of polarization affects the azimuthal direction of energy flow
in the evanescent field.
Though interference between the disturbed and reference wavefronts is the
preferred technique, direct subtraction of CCD frames liken at two phase
angles also yields increased information about lateral features in a form
of phase contrast (Block 64 in FIG. 2).
Phase shifting of an evanescent field that arises from diffraction is also
advantageous. It can be shown that the wavelength of the inhomogenous
waves in the evanescent field caused by diffraction from a normally
illuminated plane grating with subwavelength grating spacing is determined
by the spatial frequency of that grating, and so can be very small. The
phase is therefore locked to the grating structure so that altering the
grating period similarly shifts the equal phase planes in the evanescent
field. The grating can be modulated through acoustic, piezo, or other
means, as will be discussed hereinafter.
In general, evanescent illumination is also advantageous in optical
recording because of the high field strength and the smaller wavelength
evident in equation (2) above. Adding the phase shifting and detection
described above to the evanescent illumination allows even higher
information storage density as will be discussed hereinafter.
Reference is now made to FIG. 9 where it is shown that device 116 may take
on an annular geometry to coincide with the numerical aperture of a
microscope objective, as found in a photon tunneling microscope. Static
phase shift by annular phase masks is practiced in phase contrast
microscopy, except that here the masks operate at numerical apertures
greater than one so as to affect only the evanescent light. Zone 140
coincides with numerical aperture (NA) less than unity, which is the
specular light, while boundary 142 indicates the critical angle and zone
144 indicates the maximum numerical aperture of the objective. The dotted
circle 146 indicates the boundary between two annuli that cause
differential phase in a wavefront passing through them. More than two such
zones can be added, and the phase shift between the zones can be static or
dynamic, preferably under the control of computer 16. The phase shift can
arise from optical path difference, polarization, wavelength, or incident
angle, for example, with respective means of phase analysis chosen
accordingly.
In FIG. 10, the geometry of a phase shifting plate 139 is one of two
opposing slot apertures, 148 and 150. Additional slots can be added, and
the angle between the slots can be other than .pi.. Once again, the
critical angle annulus is indicated at 142, and the maximum numerical
aperture at 144. This geometry is particularly useful with polarization.
The plate 139 and slots can be static but are more useful if rotated about
the optical axis, as indicated by the arrow arcs. A typical image 152 in
FIG. 11 obtained in this way is the result of frame subtraction between
two rotational settings and shows the visualization of microspheres not
visualizable with standard photon tunneling techniques. The slotted plate
139, when rotated about the optical axis, effects a phase shift in the
evanescent field even if the slot apertures are open, by virtue of the G-H
shift, because energy flow in the lateral direction is caused to change
azimuthal direction in the sample plane.
In FIG. 12, the phase shift is accomplished by varying the incident angle,
either statically or dynamically, as between wavefronts 160 and 162, or
between either wavefront and a reference wavefront, where the critical
angle, .theta..sub.c, with respect to the optical normal, again O.sub.A,
and a total reflection boundary 164 are shown.
FIG. 13 shows a schematic of a static mask 170 for selecting specific
incident angles and therefore phase, with the critical angle annulus shown
at 172, and the maximum numerical aperture corresponding to that of an
objective at 170. Selecting annular apertures are shown at 174 and 176.
For a simple total internal reflection prism 180 as shown in FIG. 14,
wavefronts with incident angles 182 and 184 have different respective
evanescent fields and different phase angles.
Another embodiment for phase control that lends itself to dynamic phase
shifting is shown in FIG. 15. Here, the phase shift can be accomplished by
physical axial movement of the optical path 186, as controlled by a piezo
actuator 188. Phase plates and tunable optical cavities are other ways to
achieve this.
FIG. 16 is a diagram an evanescent field 190 born of a subwavelength
diffracting structure 192 with phase determined by structure geometry 194,
and with structure spatial frequency (period) 196 dynamically adjustable
for tuned heterodyning phase control. Phase shifting is thereby
accomplished with a tunable structure. The means for dynamically adjusting
the structure period can be mechanical, such as piezo, thermal, or even
humidity control, or through the use of a well-known acoustic surface
wave. Phase shifting can also be accomplished with a static structure that
has local spatial frequency variations or is moved relative to the sample
surface. Here, control of phase of the evanescent field is by illuminating
a diffractive structure with a spatial grating period 196 smaller than the
illumination wavelength so that the evanescent modes are phase locked to
the diffracting structure, and then to provide means such as, by way of
example a piezo crystal modulator or tunable acoustic wave modulator, to
modify the grating period 196 to shift the evanescent field phase. Here,
an evanescent field arises from diffraction of the incident light by a
structure with grating spatial period smaller than the wavelength of the
light such that the diffracted orders are evanescent, where the local
phase of the evanescent field is determined by and phase-locked to the
local diffracting structure element.
As stated earlier, the benefits of phase shifting and interferometry apply
to any optical device employing the evanescent field in which increased
information in the lateral spatial plane is desired. In FIG. 17, for
example, scanning near-field probe microscopes, which typically employ a
tapered waveguide or optical fiber 200 with a substantially subwavelength
tip aperture 208, utilize the evanescent field 210 thus caused to image a
proximal sample surface by scanning the aperture over the surface (not
shown). Because of the highly non-planar nature of the light from such an
aperture, the radiative components in the field are substantial. Only the
center of the field has equal phase planes 204 normal to the probe.
Therefore, restricting the phase of the light results in a smaller
effective aperture and higher resolution. Further, the lateral resolution
is dependent on the effective aperture diameter 206, which is larger than
the geometrical aperture because the metallized layer 202 of which it is
formed is an imperfect conductor and cannot contain the light.
FIG. 18 diagrammatically shows an improved near-field probe where the
aperture 220 is a multi-layer (222a-c) stack Bragg reflector which acts
upon the phase of the light to restrict the aperture. Only a few layers
are shown for clarity, although any number of layers can be used. As is
known in the art, such a structure typically consists of alternating high
and low index materials to form an energy barrier impenetrable by light of
a certain wavelength. Such a structure can be formed in many ways, however
a preferred method is to draw a preform, comprised of the waveguide with
scaled-up layer thicknesses, down to the final required size. Similarly,
the drawing process just described lends itself to forming a plurality of
such probes arranged in a close-packed matrix array.
In FIG. 19, phase shifting can be accomplished with a matrix of probes 230
moved in the XY plane. Here, the close-packed matrix 230 is locally
scanned over a small distance, so that lateral phase shifting is achieved
in addition to increasing total scan area and reducing scan time where the
matrix of probes 230 is arranged to comprise a phase-array so that
together they form a synthetic aperture.
Thus, the invention has the ability to derive lateral spatial information
about a surface illuminated with an evanescent field by controlling and
then measuring the phase of the incident and returning light,
respectively. The sample surface is the unknown, while the lateral phase
is the known.
Reference is now made to FIG. 20, which shows an arrangement by which phase
controlled evanescent field systems can be beneficially employed to
achieve high density optical reading and writing capability. In FIG. 20, a
rotating polycarbonate or other similar optical compact disk 300, or a
similar optical card scanned in the XY plane, contains data bits in the
form of optical scatter, or other optical perturbance sites 302, on or
very near the surface, or nanometer-high topographic bumps on the surface
(not shown), to which the evanescent field is sensitive. A flying total
internal reflection head 304 that can be configured as previously
described serves to illuminate disk 300 with phase controlled evanescent
field light. The information bits convert the evanescent field to
scattered light that is captured via a photodetector 306. Illumination and
phase control are provided as before via a light source 308 and phase
controller 310. A phase analyzer 312 is provided just upstream of
photodetector 306. A processor 314 is used for analyzing the signal from
photodetector 306. Alternately, the scatter sites or bumps can be internal
to the disk 300. For example, HOEs, kinoforms, micro lenses, or micro
prisms can be formed in the surface of disk 300. As can be appreciated,
phase control here greatly enhances the lateral resolution and thus
packing density for the information carrying capacity of disk 300. FIG. 21
shows an aplanatic sphere 320 that can be beneficially used in place of
flying total internal reflection head 304.
While the invention has been described with reference to particular
embodiments, it will be understood that the present invention is by no
means limited to the particular constructions and methods herein disclosed
and/or shown in the drawings, but also comprises any modifications or
equivalents within the scope of the claims.
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